1. Field of the Invention
The present invention refers to a copper pair cable, evidencing high performance when used in digital systems of the DSL (Digital Subscriber Line) type.
2. Description of the Related Art
In the past, copper pair cables were used solely for traffic consisting in voice analogical electrical signals. However, with the appearance of the Internet, those same copper cables have been used for data traffic. With the increasing demand for data traffic, and increasingly higher data rates, there came into existence the digital subscriber line (DSL), which further increases the data rate to be supported by the old copper pair cable network.
That technology makes use of digital signal processing, advanced algorithms, filters, analogical/digital converters, such as that in the ADSL (Asymmetric Digital Subscriber Line) type of transmission it is possible to reach rates up to 8 Mbps for downstream transmission and 640 Kbps for upstream transmission, thus rendering the ADSL technology particularly appealing for downloading Internet files.
However, in order that such transmission rates may be reached, it is necessary to relay the service through distances of at least 2 km, in ideal transmission conditions. There is also the problem of signal interference between the conductor pairs, known as cross talk. There is a recommendation widely used by the telephony service operators, whereby only 8 of every 25 conductor pairs of a cable may be used for traffic of a digital system, in order to avoid cross talk.
In order to solve these problems, there has been developed by means of the present invention a twisted conductor pair cable having improved electrical characteristics, whereby it is possible to reach across greater distances with the service, and also to use all the conductor pairs of a cable for digital transmission purposes.
Therefore, in order to solve the first basic problem, i.e., to allow an increase in the distance reached by the digital system, there has been made an option, according to the present invention, to reduce attenuation in the line without the need to alter the gauge of the metallic conductor. Any signal is subject to attenuation, i.e., loses power as it is carried along a line. The parameters that are responsible for this loss of power are the resistance of the conductors, the conductance (dielectric loss), the capacitance and the inductance.
Since attenuation is directly proportional to capacitance, in order to reduce attenuation in the line, the mutual capacitance of the conductor pairs was reduced, there being achieved thereby an attenuation reduced by 20% in average as compared to conventional cables, without the need to alter the diameter of the copper conductor. The reduction in capacitance was achieved by increasing the insulation thickness, since the distance between the conductors is inversely proportional to the mutual capacitance thereof.
However, in order to achieve the desired levels of mutual capacitance, using only the insulation provided by a solid sheath of polyethylene for the conductor, it would be necessary to excessively increase the diameter of the insulated conductor, that in turn would cause an increase of diameter and weight of the overall cable assembly, which would be inconvenient from both in terms of cost and in terms of installation. Although inconvenient, this solution using solid polyethylene is technically feasible. However, in order to avoid the problems incurred by increased weight and diameter, an option was made to use a combination of cellular and solid insulation.
The second basic problem in digital transmission is the maximum number of conductor pairs that simultaneously use the digital service within one same cable. An increasing number of conductor pairs carrying data will correspond to increasing cross talk.
Cross talk is the transfer of energy from a circuit (conductor pair) to another, causing loss of power of the signal being transmitted in the affected conductor pair.
In order to reduce the effect of cross talk, there are widely used twisted conductor pairs, since that when the conductor pairs are twisted with one another there occurs an effect of mutual cancellation of electromagnetic forces, causing a reduction in cross talk. Normally the conductor pairs of the cables are twisted with different pitches, a characteristic referred to as pitch series. This concept has been used for the present invention, although with a special pitch series, shorter than usual, with pitches distinct from one another, following a geometric progression, according to studies that were conducted and results that were achieved empirically.
The twisting pitches available for manufacture of the cable are those used in the running modes of the so-called twisting-cording machines. It is recommended to check each machine's kinematics system to know exactly which are the available pitches. As an example, there may be cited that in naming mode position 4-0 of the FME-3 machine, the pitches range from 71.4 mm to 197.7 mm, and the machine speed in this running mode is 95 meters per minute. If it were required to have a reduced series of pitches, there might be used, for example, running mode 1-0 of that same machine, with a minimum pitch of 26.9 mm and a maximum pitch of 74.5 mm and a machine speed of 35 meters per minute. The ratio between the two twisting pitches follows a geometric progression:an=a1·qn−1
There is not a satisfactory mathematical model for calculation of the impact of pitches upon cross talk, but it is known that shorter pitches provide better electrical performance, but however that leads to a reduction of the machine speed. It is necessary to perform a cost/benefit analysis when deciding on a pitch series.